Can you only export objects to Second Life with 2.79?
You can use the Collada exporter.

You'll find it in the File/Export menu.



Which opens to this:



If the options panel is not open, use the N key to make it appear on the right:



And that gives you access to the OpenSim presets:



Source from Machinimatrix (Gaia Clary Avastar website).